The present invention relates to processes and systems for the production of synthesis gas by steam reforming, and in particular to improved integration in such processes and systems so as to increase the recovery of waste heat, improve thermal efficiency, and eliminate or minimize air preheater corrosion.
A typical conventional steam reforming process and system shown in FIG. 1 includes feed pre-treatment 12, optional pre-reforming (not shown), steam hydrocarbon reforming 15, a waste heat recovery train for the process gas stream, and a waste heat recovery train for the flue gas stream. The waste heat recovery train for the process gas stream includes a waste heat boiler 18, a shift converter 21, a feed preheater 22, a boiler feed water heater 24, a water heater 26, a boiler feed water preparation system 32, a cooling train 29, and a hydrogen purification by pressure swing adsorption (PSA) system 35. The waste heat recovery train for the flue gas stream includes process heating coils 38, a steam generating system 39, an air preheater 42, and an induced draft (ID) fan 45.
The feed pre-treatment 12 usually involves preheating the hydrocarbon feed 11 and removing sulfur, chlorine, and other catalyst poisons from the hydrocarbon feed. The treated hydrocarbon feed gas 13 is mixed with process steam 14 and fed into the steam-hydrocarbon reformer 15 in which the mixed feed is converted to synthesis gas or process gas over a nickel catalyst bed at temperatures of 800° C. to 950° C. Heat is supplied by combusting the PSA purge gas 37 and a portion of the hydrocarbon feed 10 through multiple burners (not shown).
Heat from the process gas 17 leaving the steam-hydrocarbon reformer 15 is used to generate high-pressure steam in the waste heat boiler 18 before the process gas enters the adiabatic water gas shift converter 21. In the shift converter, carbon monoxide reacts with water and converts to carbon dioxide and hydrogen over a catalyst bed. Heat from the process gas 20 exiting the shift converter is supplied to the hydrocarbon feed preheater 22, the boiler feed water (BFW) heater 24, and the make-up water heater 26. The residual heat, usually at low temperature, from the process gas is then rejected into the environment in the cooling train 29.
The condensate 31 from the process gas resulting from the heat recovery is separated and returned to the boiler feed water preparation system 32, where the condensate 31 is combined with the make-up water 27 from the water heater 26. The combined liquid stream 33 is fed into the BFW heater 24. The heated BFW stream 30 exiting the BFW heater is sent to the steam system 49.
Finally, hydrogen product 36 is separated from the process gas in the PSA system 35. The PSA off gas 37 is returned and combusted in the reformer to supply heat to the reforming process.
The points where the temperature of one stream (heat source) gets close to the temperature of another stream (heat sink) are called “pinch points.” Pinch points reduce the temperature difference driving force for heat transfer. Therefore, a significant amount of surface area is required to recover a small amount of heat from the heat source.
More than half of the energy content in the process gas 20 exiting the shift converter 21 is the heat of moisture condensation. Unfortunately, the condensation exhibits a pinch as the process gas cools down. The pinch limits the ability to recover the heat from the process gas. As a result, a significant amount of residual heat from the process gas is rejected into the environment through the cooling train 29. Depending on the process requirement, the heat rejection could be about 20% to 25% of the total heat contained in the process gas stream exiting the shift converter.
The sensible heat from the flue gas 16 leaving the steam-hydrocarbon reformer 15 is recovered by preheating the mixed feed in the process heating coils 38 and generating additional high-pressure steam in the steam generating system 39. The flue gas stream 41 exiting the flue gas boiler is continued to preheat the combustion air 48 that is supplied by the forced draft (FD) fan 47 in the air preheater 42. The temperature of the flue gas 44 leaving the air preheater is usually cooled down to about 300° F. before the flue gas 46 is released to the atmosphere through the ID fan 45. At this temperature, the flue gas still contains a large amount of energy that is more than half of the total energy lost in the reforming plant.
It is difficult to recover the low grade or low-temperature heat (<300° F.) from the flue gas because (1) there is not a sufficient quantity of the combustion air to absorb all of the available heat, and (2) corrosion problems in the air preheater require maintaining a sufficiently high flue gas temperature to avoid moisture and/or sulfur condensation. Consequently, a significant amount of heat is rejected into the environment.
U.S. Pat. No. 3,071,453 (James) discloses a hydrocarbon reforming process in which steam is generated from the process gas waste heat at a pressure between 25 psig to 100 psig. The low-pressure steam is then super-heated and expanded in a steam turbine to generate power that drives the product gas compression. As a result, the reform process produces a high-pressure hydrogen-containing gas stream in a more efficient manner. The process utilizes the thermal energy available in the hot reformed gas to eliminate or reduce the external power requirement for product compression.
U.S. Pat. No. 3,532,467 (Smith, et al.) teaches how a steam turbine and a steam reformer can be integrated to maximize the heat recovery through steam usage. The process utilizes high-pressure steam (400 psig to 1600 psig) to drive a hydrogen-rich gas centrifugal compressor. The steam exhausted from the steam turbine at 50 psig to 350 psig is used as process steam for the steam reforming reactions. The process gas from the steam reformer is passed through the waste heat boiler, high-temperature shift, and low-temperature shift to convert most of the CO to CO2. The process gas containing mostly hydrogen, CO2, and water is cooled in a cooling train including a low-pressure steam generator and a water cooler. The gas is separated from the condensate before entering the centrifugal compressor.
The waste heat from the process gas after the shift converter is recovered by generating low-pressure steam at about 40 psig. The patent (Smith, et al.) suggests use of the low-pressure steam in the CO2 removal system. If the use of the low-pressure steam is limited to the requirement of the CO2 removal system, significant low-temperature waste heat may still be rejected to the environment through the cooling train.
U.S. Pat. No. 4,576,226 (Lipets, et al.) suggests several options to eliminate the air corrosion problem in the air preheater: (1) heated air recirculation with a forced draft (FD) fan, (2) air by-pass, and (3) preheated cold air with low-pressure steam extract from a steam turbine. Although these options are feasible to eliminate the corrosion, each option has one or more disadvantages.
For example, the heated air recirculation option requires a FD fan, power, and associated equipment. It also reduces the heat transfer performance of the air preheater. Therefore, to achieve the same heat recovery from the flue gas, more heat transfer surface area must be added to the air preheater.
The use of low-pressure steam from a steam turbine to preheat the cold air would suffer energy loss or power loss from the turbine and would also recover less heat from the flue gas if no additional heat transfer surface was added.
The air by-pass option suffers heat loss due to less heat recovery from the flue gas. U.S. Pat. No. 2,320,911 (Cooper), which controls cold air flowrate by a damper to maintain metal temperature above the flue gas dew point, suffers from the same problem.
U.S. Pat. No. 4,693,233 (Meith, et al.) discloses the use of a tubular type air preheater in which the hot flue gas flows in the tube side and cold air is in the shell side. The flue gas inside the tube is maintained at a superficial velocity of 10 ft/sec. to 100 ft/sec. Heated air is recirculated to maintain metal temperature in such a way that the droplets formed on the inside of the tube are sufficiently small and can be removed by the high velocity flue gas. As a result, no large droplets or condensation flow occurs in the tube. The high gas velocity, however, would require more fan power. The heated air re-circulation would suffer the same disadvantages described above. The control of metal temperature to generate small droplets is critical and complicates the air preheater design.
It is desired to have an integrated steam reforming process and system which maximize the use of low-pressure steam and increase the recovery of waste heat from the flue gas to result in an improved overall thermal efficiency relative to the prior art.
It is further desired to eliminate or minimize corrosion in the air preheater of the steam reforming process and system.
It is also desired to have a steam reforming process and system which afford better performance than the prior art, and which also overcome many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.